*oO 'theory of ' Cryftallifatiori, 



Polynomous Pctunze (Fig. 56) *. 



Spath etincelant oil feld-fpath en prifme a dix pans tivec <te/ 

 jommets h deux faces et quatre facettes. Daubenton Tab. 

 Miner, edit. 1 792, p. 4, vaf. 2. 



Geomet characl. Refpedlive inclination of the narrow 

 planes o n k w, cfh g, to the adjacent planes on each fide 

 150°! of the planes c t F g, PomN to thofe contiguous 

 to them by the edges /F, PN 120°; of the heptagon 

 pGcldez to' the enneagon B z e b no~Pfs, 99 41' 8"; 

 of the trapezium daft both to the plane nbafhilk and 

 to the heptagon pGtcdez, 135 ;' of the facet deab or 

 AB zp to the fanre heptagon, 124 15' 15''. 



I have not yet obferved the petunze naturally cryftallifed 

 under its primitive form. This form, fuch as it is given by' 

 the mechanical divifion' of fecondary cryflals, is that of an' 

 oblique prifm of four planes [frg. 58), two of which, fuch as' 

 GOAD, RBHN, are perpendicular to the bafes AD'NHy 

 O G R B. The other two, viz. B O A H, RG D N, make; 

 with the former, angles of 120 at the ridges O A, R N, and 

 angles of 6o° towards the oppofite ridges BH, GD. Thefe 

 planes are inclined to the bafes at the place of the ridges 

 GO, B R y 1 1 1° 29' 43", and at the oppofite ridges 68° 30' 

 J 7". 



This form is at the fame time that of the molecula. 

 Theory {hews that the two parallelograms GOAD, OGRB, 

 . as well as their parallels, are equal in extent ; and that the 

 parallelogram B O A H, or its oppofite, RGDN, is double 

 each of the preceding; which may ferve to explain the 

 roughnefs of the feftions made in the direction B O A H, 

 when compared with thofe obtained in the directions of the 



* I have adopted the name petunze, which is that given to this iub- 

 ftance in China, where it is employed in making porcelain. The word 

 fpar (fpatb) has become fo va^ut, by the application of it to fubftancea 

 very different in their nature, that it is much to be wifhed that it were 

 banifhed from the nomenclature of minerals. 



' fmaU 



